The disclosure relates in general to memory subsystems. In particular, the disclosure relates to methods and systems for generating a latch clock used in memory reading.
Among many types of random access memory (RAM) architectures, two common types of volatile RAM are dynamic RAM (DRAM) and static RAM (SRAM). Each storage element (or “bit”) of SRAM is constructed using a flip-flop, a basic storage device typically requiring approximately six transistors. In contrast, DRAM requires only one transistor per bit and is therefore generally cheaper and more compact than SRAM. SRAM is typically faster than DRAM and much simpler to use. Consequently, SRAM is typically used in computer devices that require fast but small memories, while DRAM is typically used when a large amount of RAM is needed. In fact, many microprocessor-based systems use a combination of DRAM and SRAM, constructing the large main memory from DRAM chips and using SRAM for the smaller memory devices, such as memory caches for the processor. A number of DRAM architectures are available, including synchronous DRAM (SDRAM), extended data output DRAM (EDO DRAM), and Rambus™ DRAM (RDRAM).
Conventional DRAM chips receive a plurality of input signals which define parameters such as the location, or address, of the memory data and which transmit the memory data. A read or write transaction with a DRAM generally involves two steps. First, address and control signals are asserted to the DRAM, allowing the DRAM to prepare for the data transfer. Second, the DRAM reads or writes the data, completing the data transfer. Synchronous DRAM operates similarly to conventional DRAM, although SDRAM signals include a reference clock signal to which the other SDRAM signals are synchronized. SDRAM also typically supports pipelining, which allows the SDRAM to accept address and control signals for a one memory transaction while transacting a previous memory request via the data signals.
An important consideration for selecting a memory device in a microprocessor-based system is the speed at which data can be written to and read from the memory device. Memory speed is also commonly known as “bandwidth”, a term which refers to the frequency content of the memory signals. Generally, a higher speed memory device is more efficient, because it can supply data at a faster rate (or “higher bandwidth”). In fact, memory bandwidth is often considered a crucial factor in evaluating the performance of processor-based systems. The emergence of new memory technologies and improvements in existing memory architectures are helping to increase current memory bandwidth, thus improving computer performance.
Memory timing accuracy is very important for memory access. If the timing requirements are not met, the memory device may not function properly, and the memory read and write data may contain errors.
Achieving signal timing accuracy can be very difficult in practice. The natural laws of electromagnetics which govern the transmission of electronic signals tend to introduce various amounts of delay and distortion to high frequency signals. These “electromagnetic” effects are commonly known as capacitive (or inductive) loading, which causes signal delay, and ringing, which causes signal distortion. In addition to the signal bandwidth, the physical location and dimensions of the signal conductors (known as signal “traces”) also affect the transmission behavior, as does the level of current used to transmit the signals.
Under capacitive loading, certain signals may travel across the circuit board at a delay with respect to the synchronizing clock signal. It is common to adjust for signal delays by inserting delay buffers, devices which delay electronic signals, into the memory interface. Often, each signal transmitted or received by the memory controller will be delay-buffered. By setting the proper amount of delay for each buffer, the memory signals can be synchronized with the clock signal.
A significant problem with using delay buffers is that the optimum delay can be difficult to predict before the circuit board is manufactured. First, different memory technologies may include different signals or may operate at different speeds. To accommodate a wide variety of memory devices, the delay buffers must be specially configured for each new memory system design. Even if the same type of memory device is used in a different computer system, however, differences in signal traces on the circuit board can change the required delay settings considerably.
Another problem is that, due to the inherent limitations of present integrated chip and circuit board technology, the capacitive loading can fluctuate slightly from board to board during manufacturing. Thus, even a carefully designed circuit board may not meet memory signal timing requirements due to manufacturing imperfections.
U.S. Pat. No. 6,137,734 discloses an automatic configuration of delay parameters in a dynamic memory controller. It automatically searches various combinations of transmit and receive delay pairs to determine whether successful memory read and write operations are achieved if the memory controller is configured according to each particular delay pair. The resulting collection of tested delay pairs, having either successful or failed results, may be arranged according to the delay values to form what is known in the art as a “shmoo” plot. These delay pairs, although a memory controller configured according to them will communicate successfully with its SDRAM, are close enough to failing delay values that small changes in operating conditions may cause a memory controller choosing such an operating point to fail to synchronize with its SDRAM device. U.S. Pat. No. 6,137,734 discloses an algorithm to select an optimal delay pair. This algorithm, however, may still choose operating points close to the edge of the shmoo plot.